Bacteriostatic and fungistatic additive in masterbatch for application in plastics, and method for producing same

ABSTRACT

Disclosed is an additive of ZnO/Ag° with a bacteriostatic and fungistatic action in the production of plastic items based on resins such as polypropylene, polyethylene, polystyrene, high-impact polystyrene, polyurethanes, etc., especially for producing automotive products, electrical appliances, products for washing, for storing and conducting water, for packaging, etc. The additive is prepared by mixing nanometric silver concentrates and pre-treated zinc oxide with a polymer resin compatible with the target application as a vehicle, obtaining pellets of the additive in a ratio of between 10:90 and 90:10 in weight of ZnO/Ag° : vehicle. The additive is biologically effective at a ratio in the final product of between 0.01 and 0.5 wt. % of polymer mixture and does not have any negative effects on other properties of the polymer, such as the whiteness thereof.

TECHNICAL FIELD

This invention refers to additives used in the plastics industry, particularly refers to an additive concentrate in the form of masterbatch with bacteriostatic and fungistatic activity manufactured by mixing a concentrate of nanoparticles of zinc oxide and a concentrate of metallic silver nanoparticles and a method for its manufactures.

BACKGROUND OF THE INVENTION

People have been concerned for a long time to protect themselves from germs that can cause disease, infections and growth of bacteria and moulds. The manufacture of articles with antimicrobial additives helps to increase the protection against these microbes. Many products containing antimicrobial additives exist, including soaps, lotions and disinfectants and environment refreshing products. These products are used to clean surfaces that may have germs on them, which offers some protection benefits but do not prevent microbes to settle back. In the field of health care, medical devices and dental instruments as well as several surgical implants are manufactured with antimicrobial additives. These devices are in themselves impermeable to germs, but to thoroughly protect a surface, it should be possible to replace the entire surface with a new surface including antimicrobial additives.

In the literature have been described polymeric articles with antimicrobial properties. Such articles are made in various shapes and dimensions, such as granules, films, fibers, containers, pipes, structural components, medical devices, etc. It is also known that certain metals such as silver, copper and zinc and its compounds are effective as antimicrobial agents. There have been many attempts to use this attribute in polymeric articles.

For example, in U.S. Pat. No. 5,180,585 (Jacobson et al., 1993) an antibacterial composition and a method for its manufacture are described, comprising an inorganic particle with a first coating that provides antimicrobial properties and a second coating which provides a protective function, for incorporation into polymeric materials with antimicrobial properties. In the same patent some historical references on use patterns are presented; thus in U.S. Pat. No. 4,906,466 (Edwards et al, 1990) an antimicrobial composition is described comprising a silver compound selected from AgCl, AgBr, Ag2CO3 and Ag3PO4, deposited on a physiologically inert particle, selected from oxides of Ti, Mg , Al, Si, Ce, Hf, Nb and Ta, calcium hydroxyapatite and barium sulfate. It is disclosed that the compositions may be modified by the inclusion of other ingredients such as dispersion aids, and these compositions may be incorporated into polymeric materials in an amount of 5-60% by weight of compound. The antimicrobial silver compound in contact with the polymeric article may interact with it producing undesirable effects, such as darkening due to reduction to metallic silver.

Several patents describe antimicrobial compositions in which zeolite particles as supports for antimicrobial metal ions are used. Zeolites are aluminosilicates, of either natural or synthetic origin, which have sites at which cationic exchange may occur.

When treated with solutions of metal ions, an antimicrobial metal ion can be substituted in the zeolite structure. Polymeric articles having antimicrobial properties are made by incorporating the treated zeolites with the polymer or else, the zeolite can be mixed with the polymer and then treated with a solution of the desired antimicrobial metal ion. There are no barrier coatings on the particles to prevent interactions of the metal ions with the polymer to control the rate of release of the antimicrobial species or to facilitate dispersion of the particles into the polymer article. For example, the use of zeolite particles in polymer articles is described in detail in U.S. Pat. No. 4,775,585 (Hagiwarn et al., 1988), and more specifically, U.S. Pat. No. 4,525,410 (Hagiwara et al., 1985) is directed to applications in fibers. Furthermore, it is recognized that zeolite powders tend to agglomerate and are inferior in dispersibility when mixed with resins. U.S. Pat. No. 4,741,779 (Mita et al., 1988) discloses the addition of fine silica, dry or in sol form, to provide a zeolite powder having free flowability and low agglomerating properties. Such problems as aggregation and colour development in zeolite antimicrobial compositions also are addressed in other documents which relate to the use of additives such as fatty acid salts to aid in the dispersion of UV light absorbers to prevent color development. U.S. Pat. No. 8,105,688 (Liu, 2012) meanwhile, describes a combined solid phase inorganic powder, a masterbatch, a method for manufacturing the same, and a method for manufacturing a fiber that includes, in percentage: 10-20% of the masterbatch and 80-90% of a long chain polymer; the silver content in the fiber of the present invention reaching 3-10%, with a high kill rate, UV high blocking rate, is durable and safe efficiency and having no side or toxic effects.

Another alternative for providing antimicrobial properties to the surfaces of various articles is disclosed in U.S. Pat. No. 8,282,951 (Redler, 2012), in which protective surface coatings and a method are described for applying such a coating, for example, to the external surfaces of a building. In one of the embodiments of the invention, an antimicrobial coating which includes an inorganic antimicrobial additive, a colloidal polymer medium and water is described; the additive includes particles having a diameter of 2 to 3 microns which include one element of the group consisting of silver, platinum, gold, palladium, copper, zinc, boron and a compound of any of them. The colloidal polymer medium has a polymeric resins with a size similar to or smaller than the additive particles, preferably between 0.005 microns and less than 1 micron in diameter; the preferred polymer medium being a polyurethane, an acrylic or a polyester medium.

U.S. Pat. 6,544,536 (KraII et al., 2003) discloses processes for producing plastic bodies that can be processed further for medical uses, which exhibit an antimicrobial effective content of metal compounds of special type, from which an oligodynamic effect is known, such as silver, copper and gold, as well as heavy metals such as zinc and even lanthanides, which have, as intended in the invention, an effect on bacteria and fungi because they eliminate and prevent their multiplication and their adhesion or embedding in the plastic, or having at least a long lasting effect. The difficulty to find such materials commercially lies mainly in the high cost of the quantities of metal and/or metal compounds necessary to achieve the desired purpose, particularly in the case of silver, as these substances should be included in the plastic in powder form, in which case the lower limit of efficiency of metal/metal compound is often established set in the order of 1% by weight of the plastic, being used however higher amounts to make it more effective. Some examples of these applications are to be found in U.S. Pat. No. 4,054,139; WO-A-84/01721; EP-A-0190504 DE-A-: 37 25 728, EP-A-0251783 and DE-A-39 42 112. One way to avoid high costs is not to subject all the plastic to antimicrobial treatment, but simply to coat the finished objects by any of the conventional methods such as physical vapour deposition, cathodic spraying (sputtering), plasma assisted vapour deposition, chemical methods such as currentless electroplating, reactive vapour deposition, reactive cathodic spraying, CVD, PACVD, which work only on the surfaces exposed to the source of the active substance to be applied, or as in the case of plasma assisted methods, at least one surface being open to the atmosphere being covered. The problem is solved in this patent using a method in which plastic bodies having a content of one or more oligodynamically active metals are produced, as a substance that is embedded in the plastic in the form of discrete particles, wherein the amount of active substance is not greater than 1.0% by weight, preferably not more than 0.5% by weight based on the total weight of the plastic body, and the maximum particle size of the active substance is less than 500 nm.

Certain polymeric materials, for example polyvinyl chloride, are susceptible to biological attack, especially from fungi. In order to reduce or to avoid the degradation of plastics resulting from such biological attack, biocidal products have been developed for incorporation into plastics materials or paints to kill the responsible fungi or bacteria or to prevent their proliferation. It is also known to incorporate biocidal compounds into plastics to prevent the growth of bacteria or fungi to be found in food products. In U.S. Pat. No. 5,433,424 and Japanese JP-A-06287403 and JP-A-07 071 869 among others, examples are described of this latter type of product; and items such as surfaces for food preparation and household containers etc. are already widely available to consumers.

WO 96/29361 patent application describes a biocidal polymeric matrix formed by a support matrix, an antimicrobial agent and a carrier agent, wherein the carrier agent and the antimicrobial agent are adapted to form at least one hydrogen bond or salt bridge between them.

WO 96135205 patent application describes a white cover for piano keys formed by injection of a homogeneous dispersion of a methyl methacrylate resin and 0.5-2% of an antibacterial agent comprising a ceramic body prepared by sintering a mixture of phosphate calcium and metallic silver.

WO 98/21253 patent application describes polymers which have antimicrobial properties and copolymers of vinyl monomers with non-functional vinyl comonomers of specified composition with ionic functionality.

WO 96/22023 patent application describes the use of derivatives of 2-alkyl or 2-aralkyl benzisothiazolin-3-one as fungicides for plastic materials.

JP-A-08257493 patent application discloses the use of a steel plate having a layer of paint incorporating inorganic aggregates containing anti-bacterial compounds, as a lining for refrigerators, freezers or heating cabinets.

The JP-08145394 and JP-A08145392 patent applications describe the use of plastics incorporating anti-microbial compounds for use in ventilation apparatus.

EP-A-606 762 patent application discloses a composition of a styrene polymer, an antibacterial agent and a compound having a specific functional group to produce an antibacterial resin composition.

U.S. Pat. No. 4,533,435 patent application describes an antimicrobial paper for packaging surgical supplies which incorporates antibacterial compounds in a vinylic polymer binder.

The incorporation of antimicrobial agents is reviewed by D. Smock in Formulating and Compounding Plastics, March/April 1997 p. 16 and Plastics World March 1992 p. 58.

The invention disclosed by U.S. Pat. No. 6,632,855 patent application (Beverly et al, 2003) is directed towards improving the biocidal activity of acrylic materials containing known biocidal compounds. The selection of any particular biocide for articles of the invention is made according to the end use of the article and the properties of the product, for example its activity against certain types of microorganisms, toxicity, processability, etc. The biocide is preferably present in a concentration of at least 0.25% by weight, more preferably at least 1% by weight of the polymer, for example 0.5-3% by weight. The plastic material of the invention can have many applications. It is useful as a resin for moulding or extrusion applications, for example to make doors or panels for interior or exterior applications etc. It can be supplied as a sheet material, for example, for providing walls, linings etc. or it may be suitable for forming into articles such as bathtubs for example by thermoforming. It may also be useful in the form of a primer, for example a polymethyl methacrylate resin dissolved in methyl methacrylate and optionally with a dispersion of fillers, colours and other functional particles for the manufacture of sinks, worktops, shower trays, etc. The plastic material of the invention may be especially useful as a coating on a substrate. One advantage of this form of the invention is that a relatively small amount of active biocide plastic can be used to confer a biocidal function to the surface of a non-biocidal substrate. Recommended biocidal compounds include triclosan, compounds based on heavy metals, especially silver, on inorganic carriers such as zeolites, hydroxyapatite, zinc oxide, titanium dioxide, zirconium phosphate, isothiazolones, derivates from benzisothiazolin-3-one, 10, 10 ‘oxibisphenoxiarsine, isothiazolines, zinc pyrithione, folpet (trichlormetil tio-phthalimide). Examples of biocidal compounds which are effective in the invention include those sold under the trademarks DENSIL™ S (2356 tetrachloro-4 (methylsulphonyl) pyridine from Zeneca Ltd), SK-NOB-Z.™. (Zirconium phosphate which contains silver, Sanai of Japan) and VANQUISH™ (n-butyl 1,2-benzisotiazoline, Zeneca Ltd).

From the above examples, it can be seen that no efficient and inexpensive method, is already available to enable the production of plastic articles with antimicrobial properties, especially fungistatic and bacteriostatic, producing no adverse side effects.

OBJECTS OF THE INVENTION

In view of the limitations and disadvantages of the additives with biological action in articles made from plastic resins, it is an object of the present invention to provide an additive with fungistatic and bacteriostatic activity for use in the manufacture of plastic articles.

It is another object of the present invention to exploit the biological, physical and chemical properties of metallic silver and zinc oxide in the preparation of an additive with bacteriostatic and fungistatic activity for use in the manufacture of plastic articles while not deteriorating the desired properties of the material.

It is still an object of the present invention to provide an additive comprising metallic silver and zinc oxide nanoparticles.

It is yet another object of the present invention to provide an additive with fungistatic and bacteriostatic activity in the form of pellets masterbatch, for easy handling and integration with the polymer to be used in the final application.

Still another object of the invention is to provide an additive that is effective when it is used in a proportion of between 0.01% and 0.5% on the total weight of the plastic formulation, depending on the type of polymer and its final application.

It is yet another object of the present invention to provides a fungistatic and bacteriostatic additive which does not migrate to the surface of finished plastic articles.

It is yet another object of the present invention to provide an additive that is not degraded by ultraviolet rays.

These and other objects of the invention will be apparent from the following description and accompanying figures.

BRIEF DESCRIPTION OF THE INVENTION

In the text that follows, some of the terms used have the meaning described below.

“masterbatch”: a concentrate containing ingredients that produce specific desired performance benefits either in the manufacture of the product or in the final product itself.

“bacteriostatic”: means a substance that prevents cell reproduction of bacteria and therefore their proliferation.

“fungistatic” is a substance that prevents or inhibits cellular reproduction of fungis.

The present invention relates to an additive having fungistatic and bacteriostatic activities for the preparation of plastic items, especially for items that have direct contact with substances to be ingested or which come into direct contact with the skin, for instance drinking water reservoirs, cleaning accessories as well as tools and equipment for personal use.

The additive is a concentrated masterbatch in pellet form, prepared starting from a mixture of concentrates of zinc oxide and silver oxide obtained from a paste of nanometric zinc oxide which is given a surface treatment to improve their properties for integration into the final compound and to prevent its degradation, as well as a paste of nanometric metallic silver.

Due to the characteristics acquired by the additive as a result of the manufacturing process, the additive may be incorporated directly and homogeneously to the plastic material to which it is added, ensuring the same activity across the exposed surface, and being integrated homogeneously in the polymer, the active agents (silver and zinc oxide) not being exhausted by migration to the surface, which extends the useful life of the item as to its bacteriostatic and fungistatic characteristics.

Due to the non migration of the particles it is sufficient to provide an article with an active surface containing said active compounds, making it possible to use plastics processing techniques in which the treated polymer occupies the outer layer or external surface of the article, with thickness of between 40 and 50 microns, thus being preferred processes of coextrusion, coinjection and rotomoulding for the production of articles.

The homogeneity in the integration of zinc oxide paste is ensured by a surface treatment of the nanoparticles with additives based on bifunctional a carbon hydrolyzable silane, for its affinity with a wide variety of inorganic substrates and its integration into a polymer carrier prior to mixing with a concentrate of nanometric metallic silver mixed with a thermoplastic polymer that acts as vehicle.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram showing the process for producing the additive which is the object of the invention.

FIG. 2 is a graph showing the comparative results of microbial reduction for 6 strains of organisms, for variable concentrations of ZnO/silver additive.

FIG. 3 is a graph showing the effect of the concentration of the additive on the whiteness (L*) in containers with different contents of ZnO/Ag°.

FIG. 4 shows the comparative results using the additive at concentrations of 0.0, 0.1 and 0.5%, in different polymer matrices on microbial activity for three strains of bacteria.

FIG. 5 shows the comparative results using the additive at concentrations of 0.0, 0.1 and 0.5%, in different polymer matrices on microbial activity for two strains of fungi.

FIG. 6 shows the effects of using the additive in polypropylene copolymer at concentrations of 0.15 and 0.3% before and after aging in an UV chamber.

DETAILED DESCRIPTION OF THE INVENTION

The present invention refers to an additive with bacteriostatic and fungistatic activity containing nanoparticles of ZnO/Ag°, in presentation of masterbatch in the form of pellets, used in the manufacture of plastic-based resins such as polypropylene, polyethylene, vinyl polychloride, polystyrene, high impact polystyrene, polyurethanes, etc. and the process for preparing the same.

The additive of the invention is directed to the manufacture of polymer products requiring to offer antimicrobial activity, for instance automobile parts, textiles, home appliances, toilet items, storage and transportation of water, packaging, etc.

In FIG. 1 the preferred production method for the fungistatic and bacteriostatic additive of the invention is shown, the method comprising the following process steps:

I. Preparation of a Concentrate of Zinc Oxide.

1. To mix a paste of nanometric zinc oxide (100) coming from a surface treatment based in silanes, in a high intensity mixer (300) (for example, a mixer sold under the trademarks Draisswerke, Brabender, etc.) with the polymer or polymers which are compatible with the resin of the final application (200), until a 20% by weight concentrate of zinc oxide is obtained.

2. To mix the concentrate from the previous step to between 1 and 10% by weight, with a resin (500), of the same nature as that of the previous step in a twin screw extruder (400) (such as the type sold under the trademarks Brabender, Coperion, Werner & Pfleiderer, etc).

The mixing of step 2 with a resin compatible with the application to which the additive will be destined, permits said resin to act as a vehicle that can be easily integrated into the target resin and the percentage of concentrate added to the mixture should be such as to permit obtaining the desired ZnO:Ag° relation to incorporate into the final product.

II. Preparation of a Concentrate of Nanometric Metallic Silver.

3. To mix (350) a paste of nanometric metallic silver (150) with a thermoplastic polymer (250) that is compatible with the resin of the final application, to obtain a concentrate of 10% silver,

4. To mix (450) the resulting mixture in a twin screw extruder (for example, the type marketed under the Brabender, Coperion, Werner & Pfleiderer, etc. trademarks) with a resin (550) of the same nature as the resin of step above, until obtaining a paste with between 0.1 and 1% silver

The mixing of step 2 with a resin compatible with the application to which the additive will be destined, permits said resin to act as a vehicle that can be easily integrated into the target resin and the percentage of concentrate added to the mixture should be such as to permit obtaining the desired ZnO:Ag° relation to incorporate into the final product.

III. Preparation of the Additive.

5. To nix the zinc oxide concentrate from step I with the silver concentrate resulting from step II, in a twin screw extruder (700) (for example, the type marketed under the Brabender, Coperion, Werner & Pfleiderer etc. trademarks), for obtaining the additive concentrate of ZnO/Ag° in masterbatch in pellet form.

Preferred Mode of the Invention

To achieve the optimal effect of the additive of the invention, as to maximize the biological effect and to reduce the possible effects on the properties of the resins in the final article, it is important to control the raw material specifications and the conditions of the method. In the following the data reflecting the best performance of the additive are offered.

Raw Material

Table 1 shows the preferred ranges for the specifications for the ZnO paste to be used in preparing the additive of the invention.

TABLE 1 Specifications for ZnO paste Characteristic Specification Method Purity (%) 100 DRx Size of the particles D₁₀ (micron) 0.04-0.09 Horiba D₅₀ (micron) 0.050-0.087 Horiba D₉₀ (micron)  0.1-0.13 Horiba Solid contents (weight %) 45-50 Gravimetric

Table 2 shows the preferred ranges for the specifications for the Ag° paste to be employed in preparing the additive of the invention, taking as an example the paste obtained from the method described in WO 2008/075933 patent application (Martinez et al, 2008).

TABLE 2 Specifications for Ag° paste Characteristic Specification Method Purity (%) 99-99.3 Gravimetric Size of the particles D₁₀ nm 41.5 Coulter LS230 D₅₀ nm 47.0 Coulter LS230 D₉₀ nm 56.0 Coulter LS230 Solid contents (weight %) 80-85  Gravimetric

Examples are provided in the following for the preparation at laboratory scale of the additive of the invention.

EXAMPLE 1 Preparation of Raw Material Concentrates

A. Preparation of the silver concentrate in low density polyethylene (LDPE).

To prepare the silver concentrate an intensive mixing equipment for polymers is used.

-   -   Step 1. Enter into the equipment 167 g of silver paste with 80%         by weight of solids,     -   Step 2. Add 1200 g of virgin LDPE resin to obtain a concentrate         of about 10% by weight of silver,     -   Step 3. Mix for 8 minutes ensuring the complete integration into         the molten polymer,     -   Step 4. Homogenize the material obtained by means of a twin         screw extruder,     -   Step 5. Check final concentration, which should be about 10% by         weight.

B. Preparation of the ZnO Concentrate in Low Density Polyethylene (LDPE)

It is started from a paste of zinc oxide at 50% by weight solids with the above described specifications.

Once the dough is obtained its integration into the polymer is carried out in an equipment for intensive mixing of polymers, performing the following steps:

-   -   Step 1. Introduce 600 g of paste of zinc oxide surface treated         with silane additives into the chamber of the equipment,     -   Step 2. Introduce 1200 g of LDPE resin,     -   Step 3. Mix for 15 min to ensure good integration of the paste         of zinc oxide nanoparticles with the virgin resin obtaining a         concentrate of 20% zinc,     -   Step 4. Homogenize the material by means of a twin screw         extruder.

Example 2 Preparation of 5 kg of the Concentrate of ZnO/Ag° LDPE at 5% for a ZnO/Ag Ratio of 90:10.

-   -   a. Mix 225 g of zinc oxide concentrate with 25 g of silver         concentrate, prepared according to Example 1,     -   b. For a final application of 1 kg of LDPE, 40 g of the mixture         from the previous step will be added, diluting with 960 g of         LDPE for final application in a twin screw extruder for the         additive ZnO/Ag° to be 0.2% by weight of the final mixture.

Example 3 Preparation of 5 kg of the Concentrate of ZnO/Aq° LDPE at 1% for a ZnO/Aq Ratio of 90:10.

-   -   a. Mix 45 g of the zinc oxide concentrate with 5 g of silver         concentrate, prepared according to Example 1,     -   b. For a final application of 1 kg at 0.05%, 50 g of concentrate         will be added, diluting 950 g of the final compound for a final         application in a twin screw extruder for the additive ZnO/Ag° to         be at 0.05%.

It is to be noted that the above examples produce an additive in a ZnO/Ag° ratio of 90:10, which is the recommended ratio for best results in typical applications, however combinations up to 10:90 ZnO/Ag can be used passing through the intermediate ranges for different performances and applications.

Results

The additive obtained according to the described method was tested on several polymers, in accordance with Examples 2 and 3, which were exposed to three bacterial strains and four fungal strains with the results shown below.

In Table 3 are shown in summary, the comparative results obtained in the assessment of the effects on microbial activity (Drop Test) for strains of Staphylococcus aureus, Escherichia coli, and fungi Trichoderma viride, Aspergillus niger, Candida albicans and Mycosphaerella fijensis in low density polyethylene (LDPE) for use in packaging.

FIG. 2 is a graph showing the comparative results of Table 3, for the percentage of microbial reduction in packaging of low density polyethylene (LDPE), on the vertical axis, where the first group of columns corresponds to LDPE without additive and the following six groups of columns refer to LDPE with 0.0 and 0.05% ZnO additive in relationships ZnO:Ag° from 10:90 to 90:10; the columns correspond, from left to right to Staphylococcus aureus, Escherichia coli, Trichoderma viride, Aspergillus niger, Candida albicans and Mycosphaerella fijensis.

TABLE 3 Effects of the use of the additive on the microbial activity in LDPE (Drop Test) for two strains of bacteria and four fungal strains at a concentration of 0.05% of the additive with different relationships ZnO:Ag° Microbial Reduction (%) Relationship S. E. T. A. C. M. ZnO:Ag° aereus coli viride niger albicans fijensis 0 41.5 42.9 38.5 38.7 35.4 36.6 10:90 79.1 77.6 75.8 76.1 74.3 74.0 25:75 83.0 81.1 82.9 79.5 80.2 81.4 50:50 84.8 82.0 81.6 81.5 79.7 78.5 75:25 84.2 85.7 82.6 81.7 81.3 82.4 80:20 86.1 86.4 83.7 84.2 81.3 83.8 90:10 92.5 92.1 90.3 88.9 89.1 86.7

It can be appreciated that the increased efficiency of the microbial reduction by the action of the additive is substantial for all strains. As to the increase in the ratio of ZnO in the relationship)(ZnO/Ag° a remarkable improvement in microbial reduction is observed, with the result that the 90/10 ratio ZnO/Ag° is where the highest microbial reduction is obtained, although the additive is effective even with the 10-90 ratio, compared to the reduction produced by the polymer alone.

In Table 4 is shown the effect of the content of the additive on the colour of the LDPE containers at different concentrations of ZnO/Ag°. The L* value refers to the colour settings for brightness ranging from zero (black colour) to 100 (white colour).

TABLE 4 Effect of additive concentration on the whiteness (L*) in containers with different contents ZnO/Ag° Whiteness(L*) Relationship Concentration of additive (%) (ZnO/Ag) 0 0.025 0.05 75:25 84 84 85 80:20 85 85 86 90:10 85 87 88

FIG. 3 is a graph showing these results. It can be seen that the increase in additive content from 0%, for the columns on the left, 0.025% for the central columns up to 0.05%, for the columns to the right, does not significantly affect the value L*, shown on the vertical axis, associated with the whiteness of the compound. It is also important to note that the colour is not affected by the presence of silver because of the low doses used and also by the change in the ZnO/Ag° relationship.

Tables 5 and 6 show comparative results on microbial activity, using two concentrations of the additive in various polymeric matrices, tested by the Test Drop method for three strains of bacteria (Table 5) and three strains of fungis (Table 6). The selected polymeric matrices for testing are as follows:

-   -   an EVA polymer produced by Dupont under the trade name Elvax         250®,     -   a PP polymer marketed under the brand Indelpro®,     -   a HIPS polymer marketed under the brand Resirene®

Additive concentrations are such that an efficiency is achieved in the microbial reduction of more than 90%.

TABLE 5 Effects of the use of the additive on microbial activity of three bacterial strains. Bacterial strains for the test % additive in % Microbial Reduction the final Escherichia Salmonella Staphylococcus Polymer application Coli Choleraesius Aureus EVA 0.0 11.9 11.5 10.6 0.1 91.2 90.7 92.8 PP 0.0 33.33 10 0 0.1 99.86 99.4 99.5 HIPS 0.0 81.8 79.6 78.8 0.5 99.4 98.6 96.5

From the table is also evident that the effect of the additive is significant in low concentrations, although the effectiveness depends, as is well known, on the nature of the polymer in which it is applied.

TABLE 6 Effects of the use of the additive on microbial activity for three strains of fungi. Bacterial strains for the test % additive in % Microbial Reduction the final Trichoderma Aspergillus Mycosphaerella Polymer application Viride Niger Fijiensis EVA 0.0 11.2 10.4 10.7 0.1 92 90.3 90.1 PP 0.0 50 0 0 0.1 97.5 97 70 HIPS 0.0 79.1 78.6 79 0.5 95.3 93.2 93.7

FIGS. 4 and 5 are graphical representations of the results shown in Tables 5 and 6, respectively, the vertical axis showing the percentage of microbial reduction and the horizontal axis showing the percentages of additive employed for each polymer evaluated, showing FIG. 4 , groups of three columns for E. coli, S. choleraesuis and S. aureus, from left to right, and in FIG. 5 T., viride, A. niger and M. fijiensis, respectively, making it easier to appreciate the difference in microbial reduction.

Table 7 shows the effects of using the additive at different concentrations (0.15 and 0.3%) of 340 RP polypropylene before and after aging in UV chamber, before aging for E. coli and after aging, for S. aureus.

TABLE 7 Effects of the use of the additive in RP 340 polypropylene copolymer before and after aging Before aging After 120 h of aging Sample Inhibition %. Inhibition %. PP compound — — Compound with Triclosan 98.40% 88.06% PP compound with 0.15% 97.30% 93.28% additive PP compound with 0.3% 99.20% 95.52% additive

FIG. 6 graphically shows the difference in the count of bacterial colonies, vertical axis, before and after aging (left and right columns respectively in each pair of columns) as well as the effect of the additive and the variation in the concentration. As can be seen in the table, considering the groups of sorted columns from left to right for the compound alone, the compound with triclosan, the compound with 0.15% of additive and the compound with 0.3% of additive, respectively, both the organic compound (triclosan) and the inorganic compound based on Zn/Ag° are highly efficient at inhibiting the growth of microorganisms with greater than 97% inhibition rates. However, the antimicrobial compound with the organic type additive (triclosan) starts decreasing its antimicrobial effectiveness as aging time reaches 120h, while the additive of inorganic nature Zn/Ag° remains active even after 120 h, this being due to the fact that the additive ZnO/Ag° is not consumed and does not migrate to the surface, but remains anchored to the polymer matrix and starts acting as the material wears out. After the discussions of these results it is to be concluded that some of the effects and/or advantages of using the additive of the invention are as follows:

-   -   it is a masterbatch additive in the form of pellets whose         presentation makes it easily manageable during processing and         application,     -   it is compatible with a wide range of thermoplastic resins and         “commodities” resins which are commonly used in the market for         various applications,     -   it can be used in various methods of plastics processing, either         extrusion, injection, blow moulding, thermoforming, cast film,         etc.     -   it is highly efficient in controlling colonies of bacteria and         fungi commonly found in nature,     -   it is possible to achieve high rates of microbial inhibition at         very low concentrations of additive in the final application         starting from 0.05% and it is not required to exceed 0.5% by         weight of the polymer-additive mixture,     -   the effect of migration of the active compounds is removed,         producing a lasting effect, beyond that expected for additives         with similar action, for example, organic-based additives,     -   photocatalytic reactions are prevented and therefore the         premature degradation of the polymer due to its surface         treatment in avoided, also facilitating its dispersibility and         easy integration into the polymer into which it is to be         applied,     -   it does not have negative effects on the polymer properties,         especially the colour of the polymer is not affected due to the         presence of silver due to the low doses in which it is used,     -   it is recyclable and its antimicrobial effect is not affected by         reprocessing cycles and/or recycling.

From the foregoing description and examples, it is evident that the additive of the invention offers important advantages with respect to other additives conventionally employed in particular providing greater effectiveness in controlling fungi and bacteria, while maintaining without negative effects other properties of the target material; to one skilled in the art it will be apparent that the proportions of ZnO/Ag° in the final resin mixture shown in the examples are not limiting and are only intended to show a comparison of the effect of the presence of active elements as to its bacteriostatic and fungistatic activity and non damaging effects for other properties, so that such proportions can and should be adjusted to said target material, however, the method for preparing the additive is such that ensures dispersibility and permits better control of the amount of each of the active compounds in the final mix. 

1. An additive in the form of masterbatch in pellet form, with bacteriostatic and fungistatic activity to be employed in the manufacture of plastic articles, obtained by mixing concentrates of Ag° and ZnO in paste form with a polymeric carrier, characterized in that: a) the metallic silver in the paste used for preparation of the concentrate is in the form of nanoparticles with the following specifications: Characteristic Specification Purity (%) 99-99.3 Size of the particles D₁₀ nm 41.5 D₅₀ nm 47.0 D₉₀ nm 56.0 Solid contents (weight %) 80-85 

b) the zinc oxide in the paste used for the preparation of the concentrate is in the form of nanoparticles with the following specifications: Characteristic Specification Purity (%) 100 Size of the particles D₁₀ nm (micron) 0.04-0.09 D₅₀ nm (micron) 0.050-0.087 D₉₀ nm (micron)  0.1-0.13 Solid contents (weight %) 45-50

c) the polymeric carrier is selected to be compatible with the final application in which the additive will be used, and d) the percentage by mass of ZnO and Ag° in relation to the vehicle in the additive is in the range of from 10:90 to 90:10.
 2. An additive in the form of masterbatch in pellet form, with fungistatic and bacteriostatic activity, for use in the manufacture of plastic articles according to claim 1, characterized in that, being present in a proportion of from 0.01% to 0.5% by weight in a mixture for a particular application for manufacturing plastic items, it produces a microbial reduction of bacteria and fungi higher than 97% before aging and greater higher 93% after 120 hours of aging in UV chamber.
 3. Method for the production of the additive paste according to claim 1, characterized in that it comprises the following steps: a) Preparation of the zinc oxide concentrate starting from a paste of nanometric zinc oxide which has been previously subjected to a surface treatment based on silane additives, b) Preparation of the silver concentrate starting from a silver paste of nanometric metallic silver, and c) Preparation of the additive by mixing the zinc oxide concentrate of step (a), the metallic silver concentrate resulting from step (b), and a polymeric carrier compatible with the resin for the end use in a proportion from 10:90 to 90:10, to obtain the concentrate additive of ZnO/Ag° in masterbatch in pellet form.
 4. Method for the production of the additive paste according to claim 3, characterized in that for the preparation of the zinc oxide concentrate, the following steps are followed: a) Subjecting a paste of nanometric zinc oxide at 45-50 wt % solids, to a surface treatment based on silane additives, b) Feeding the treated zinc paste to a high intensity mixer where it is mixed with the polymer or mix of polymers compatible with the final application in which the additive is to be used, c) concentrating the mixture of zinc oxide and polymer up to 20% by weight of zinc oxide, and d) Homogenizing the material by means of a twin screw extruder, mixing the concentrate from the previous step with a polymer compatible with the final application, to a concentration by weight of zinc oxide of between 1 and 10%.
 5. Method for the production of the additive paste according to claim 3, characterized in that for the preparation of the nanometric metallic silver concentrate, the following steps are followed: a) mixing a paste of nanometric metallic silver with 80-85 wt % of solids, with a thermoplastic polymer that is compatible with the final application to which the additive will be destined, in a high intensity mixer, concentrating the resulting mixture to 10% by weight of silver, b) Homogenizing the material by means of a twin screw extruder, mixing with a resin compatible with that of the final application, up to a concentration between 0.1 and 1% by weight of silver.
 6. Method for the production of the additive paste according to claim 3, characterized in that for the preparation of the additive: a) the silver and zinc oxide concentrates are mixed with a resin compatible with that to which the additive will be destined, up to obtaining a concentration of silver and ZnO of between 10 and 90% by weight of the mixture, b) the mixture is extruded to form pellets of the additive. 